This invention relates to photoactive semiconductor mixed metal oxide materials suitable for use as electrodes in electrochemical cells, photoconductors, and in "photoassisted" electrochemical reactions utilizing true solid/solid solutions of diverse metal oxides to produce the photoactive semi-conductor mixed metal oxide material. A method of producing said true solid/solid solution type of photoactive semiconductor mixed metal oxide material is also disclosed.
There has been considerable recent interest in the application of photoactive semiconductor electrodes to the electrolysis of water and to the direct conversion of solar energy to electrical, fuel, chemicals and/or chemical energy. The uses of such electrodes have been generalized as reductionoxidation reactions in addition to the electrolysis of water. Chemical reactions resulting from photoactive semiconductor electrodes can be carried out at potentials much lower than otherwise required utilizing light as an additional driving force for these reactions. Such processes may be termed "photoassisted" rather than photocatalyzed reactions. There are, however, two major obstacles which must be overcome in order to make direct conversion of solar energy to electrical, fuel, chemicals and/or chemical energy or the electrolysis of water, a viable commercial process both on the industrial level and the consumer level. The first of these two problems is reducing the cost of producing the desired end result, i.e., the electrolysis of water or direct conversion of solar energy to electrical, fuel, chemicals and/or chemical energy. The second problem is producing a system that has a long life in actual use. An acceptable life span is generally thought to be 20 years. Oxides of niobium, tantalum, titanium, and tin tend to answer both the above problems and do exhibit the necessary photochemical response. It has been long known, for example, that titanium dioxide (TiO.sub.2) fills both of the requirements of long life and economical production of electrodes for use in the electrolysis of water or the direct conversion of solar to chemical or electrical energy. However, titanium dioxide by itself has an extremely large (relative to where the energy of the solar spectrum falls) "band gap." The term "band gap" as herein and hereafter used means the minimum amount of energy needed to raise an electron in a valence band to an energy level in a conduction band. This band gap is too large for use with approximately 97 percent of the available solar energy, i.e., TiO.sub.2 absorbs wave lengths that are shorter than about 400 nanometers, and about 97 percent of the terrestrial solar spectrum has wave lengths that are longer than 400 nanometers. Titanium dioxide does have an additional advantage of being a material which is not toxic to the general environment. Thus, it does not have any of the generally harmful effects to the environment commonly associated with materials having a natural band gap more closely attuned to the solar spectrum such as, for example, cadmium selemide (CdSe) and gallium arsenide (GaAs).
It is known that electrodes fabricated from, for example, single crystals of pure titanium dioxide, doped single crystals of titanium dioxide, or polycrystalline titanium dioxide which may or may not be deposited on an appropriate substrate can be used as photoelectrodes. Titanium dioxide has a band gap which is unacceptably inefficient of about 3.0 eV. This band gap results in a maximum terrestrial power conversion efficiency of only about 1 or 2 percent.
To form electrically conductive semiconductor material, the titanium dioxide may be typically treated by reduction with hydrogen or reduction in a vacuum. It is theorized that such treatment produces a material with oxygen latices deficiencies in the titanium dioxide crystal, with these latice defect sites contributing to the semiconductor properties. This partially reduced material can be characterized by the general formula TiO.sub.(2-x), where x takes on a value of between 0 and 1. Because of the great possibilities which these electrodes have for conversion of solar energy to electrical or chemical energy, a number of studies have been directed to methods of fabricating electrodes which make such conversions more efficient. In previously described uses of n-type titanium dioxide semiconductor electrodes, it has generally been the practice to use electrodes formed from single crystals of TiO.sub.2 or a reduced polycrystalline TiO.sub.2.
The technique of producing single crystal, photoactive TiO.sub.2 electrodes is described, for example, by S. N. Frank et al in "Semiconductor Electrodes II, Electrochemistry at n-type TiO.sub.2 Electrodes in Acetonitrile Solutions," J. Am. Chem. Soc. 97:7427 (1975). Polycrystalline titanium dioxide electrodes produced by chemical vapor deposition techniques are described, for example, by K. L. Hardee et al in "The Chemical Vapor Deposition and Application of Polycrystalline n-type Titanium Dioxide Electrodes of the Photosensitized Electrolysis of Water," J. Electrochem. Soc. 112:739 (1975).
Single crystal TiO.sub.2 electrodes or doped signal crystal TiO.sub.2 electrodes are costly and difficult to produce. On the other hand, polycrystalline electrodes which utilize TiO.sub.2 as the photoactive semiconductor material are less difficult and less costly to produce but are still limited in their spectral response to wave lengths of about 400 nanometers and shorter.
Another method of trying to alter the spectral response of the TiO.sub.2 electrodes involves making physical mixtures of titanium dioxide and other compounds with optical absorption loser to the desired optimum of the terrestrial solar spectrum, see for example, U.S. Pat. Nos. 4,181,593 and 4,181,754. Said U.S. patents teach physical mixtures which are not homogeneous mixtures at the ionic or molecular level. The nonhomogeneous mixtures are limited to titanium dioxide and other metal oxides which have a chemical oxidation state other than +4 which are sintered and placed on a substrate. While this teaching does produce an electrode, it does not produce an electrode which has the necessary efficiency to make it economically feasible in the market place. Additionally, the above-identified U.S. Pat. No. 4,181,593 teaches an optical absorption adjustment of only 70 namometers at best, i.e., up to about 470 nanometers. This is still far from the optimum wavelength of approximately 800 nanometers. Yet another method used to modify TiO.sub.2 has been what is called "dying" of the TiO.sub.2 either supported by another substrate or unsupported. These systems use a film layering over the TiO.sub.2 of a material, frequently organic, which absorbs solar energy more efficiently than TiO.sub.2 alone. These systems, however, are deficient in a number of areas. First, they do not provide the longevity necessary for an economical system in the market place. Secondly, they are not efficient.
It is therefore an object of the present invention to provide a photoactive semiconductor mixed metal oxide material suitable for use, for example, as electrodes, comprising true solid/solid solutions of diverse metal oxides which are simple and inexpensive to produce, having an optical absorption which may be optimized to the solar or other spectra and which produce the necessary longevity. These and other advantages will become apparent in the following description of the instant invention.